1. Field of the Invention
This invention relates to a process for preparing an unsaturated carboxylic acid by vapor-phase catalytic oxidation of the corresponding unsaturated aldehyde with molecular oxygen, and more particularly to a process for preparing acrylic acid or methacrylic acid from acrolein or methacrolein, using a catalyst comprising phosphorus, molybdenum, oxygen, magnesium and optionally at least one of Ca, Sr, and Ba, and at least one of V, W and Ni as an optional component.
2. Description of the Prior Art
Heretofore, many processes have been proposed for preparing unsaturated carboxylic acids from the corresponding unsaturated aldelydes. For example, Japanese Patent Publication No. 12129/69 discloses a process for preparing acrylic acid from acrolein, using a catalyst consisting of Mo, V, W and silica, Japanese Patent Publication No. 19260/63 a process for preparing acrylic acid, using a catalyst consisting of P, Mo and As, etc.
Many processes have been proposed for preparing methacrylic acid. For example, Japense Patent Publication No. 6605/72 discloses a process based on a catalyst consisting of Mo, Ni and Ti, Japanese Patent Publication No. 10773/73 a process based on a catalyst containing Mo and Ti, etc. Furthermore, U.S. Pat. Nos. 3,686,294 and 3,761,516 disclose processes for preparing methacrylic acid, based on a catalyst of P-Mo-As type, U.S. Pat. No. 3,795,703 a process based on a catalyst of P-Mo-alkali metal type, and Belgian Pat. No. 817,100 a process based on a catalyst of P-Mo-Sb-Zn type.
The catalysts disclosed in these publications and patents have their respective advantages, but from the industrial viewpoint they have disadvantages in selectivity or catalyst life.